Mechanical Properties and Photomechanical Fatigue of Macro- and Nano-dimensional Diarylethene Molecular Crystals.

The diarylethene derivative, 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)perfluorocyclopentene, undergoes a reversible photoisomerization between its ring-open and ring-closed forms in the solid-state and has applications as a photomechanical material. Mechanical properties of macrocrystals, nanowire single crystals, and amorphous films as a function of multiple sequential UV and visible light exposures have been quantified using atomic force microscopy nanoindentation. The isomerization reaction has no effect on the elastic modulus of each solid. But going from the macro- to the nanowire crystal results in a remarkable over 3-fold decrease in the elastic modulus. The macrocrystal and amorphous solids are highly resistant to photomechanical fatigue, while nanowire crystals show clear evidence of photomechanical fatigue attributed to a transition from crystal to amorphous forms. This study provides first experimental evidence of size-dependent photomechanical fatigue in photoreactive molecular crystalline solids and suggests crystal morphology and size must be considered for future photomechanical applications.